Coma Physics of an Interstellar Object: JWST Spatial–Spectral Mapping of 3I/ATLAS

Abstract We report a survey of molecular emission from cometary volatiles using the James Webb Space Telescope (JWST) toward interstellar object 3I/ATLAS carried out on UT 2025 December 22 and 23 at a heliocentric distance ( r H ) of 2.37–2.41 au. These measurements of CO, CO 2 , H 2 O, CH 3 OH, and CH 4 sampled molecular chemistry in 3I/ATLAS as it receded from its encounter with our Sun and entered the vicinity of the H 2 O ice line—the region between r H = 2 and 3 au where the temperature becomes too low for H 2 O to vigorously sublime, and CO and CO 2 begin to control the overall activity. CO was the most abundant molecule, followed by H 2 O and CO 2 , whose molecular abundances with respect to CO were (40.5 ± 3.1)% and (41.6 ± 0.3)%, respectively. This work presents spatial–spectral maps of column density and rotational temperature as a function of distance from the nucleus for all detected species. The spatial distributions of both quantities were highly anisotropic for the apolar species in the coma of 3I/ATLAS, yet were more nearly symmetric for the polar molecules. These results demonstrate how volatiles were segregated in the nucleus ices of 3I/ATLAS and reveal heating and cooling mechanisms in its coma. Derived maps of the ortho-to-para ratio (OPR) for H 2 O were flat with increasing distance from the nucleus and consistent with a coma-averaged value OPR = 2.7 ± 0.2, slightly less than the expected equilibrium value of 3.